As you see when the first row is inserted then then the fist extent is created.
It als means that the if you have an index on the table this also get its first extent.
So there is a slide overhead when the first rows are inserted.

SQL> select blocks, bytes/1024/1024, segment_name, segment_type from dba_segments where segment_name like'NOAAP%';
BLOCKS BYTES/1024/1024 SEGMENT_NAME SEGMENT_TYPE---------- --------------- --------------- --------------- 8 .0625 NOAAP_2 INDEX 1280 10 NOAAP TABLE 8 .0625 NOAAP_1 INDEX
Now what is the advantage.
large applications can created a lot of segments that are not used.(SAP is one of those one's).
Tables can have many indexes that may never be used. Which can result in wasted storage.
If you don't want this then set the following parameter on false

SQL> alter system set deferred_segment_creation = FALSE;

Now does this mean when you truncate the table you don't have any extent anymore.